My personal view is that if the character is fictitious and the actor embodies the persona of the character, then race is a non issue. I don't see James Bond as having to be white, he just needs to be a well spoken (English/Scottish) attractive person who is handy in a fight or car chase. There are slight double standards at play, for example, could a white person play Shaft with any credibility? What if the actor had been adopted at birth to a black family and grew up in Harlem, is that OK? The uncommon nature of that type of casting may cause a suspension of disbelief issue for viewers, which would hurt the marketability of the product, in the same way that people lose their minds when they meet a white person with a Jamaican or Chinese accent.
As for a race of fantasy people, surely in the context of modern society it is highly improbable that they would all be white? Tolkien grew up in a majority white society which probably shapes our perception of what a Hobbit should look like, and he characterised Orcs as being a darked skinned foreign evil. But lets be honest, making a huge series where every character with a dark complection is evil and most with light complections are good is not really a progressive message, nor is it actually important. This isn't a case of making Anne Boleyn black, it's also about giving jobs to talented (OK that is a separate debate re. Lenny) actors where skin colour is not a defining characteristic of a fantasy character.